A vehicle door structure used as a side door of a motor vehicle is formed of an outboard outer panel and an inboard inner panel. For reinforcement, a door structure is typically fitted with a reinforcement beam called as door beam having a front and rear end connected to the front and rear part of the inner panel, respectively. There are various forms of door beams, and they include those using a steel channel member that forms a box-shaped cross section in cooperation with the inner panel, those using a double-hat shaped cross section and those using a pipe member. See Japanese patent No. 3989373, and Japanese patent laid open publications No. 2005-126018 and No. 2006-021744.
A door beam having such a cross section reduces the deformation of the door opening of the vehicle body at the time of a frontal crash by transmitting the crash load (fore-and-aft axial load) from the front pillar (A-pillar) to the center pillar (B-pillar), and protects the inner space of the cabin by absorbing the impact load at the time of a side crash by causing a plastic deformation by means of the collapsing of the cross section.
The conventional door beam typically consisted of a member separate from the inner panel, and there is an overlap between the inner panel and door beam in the area where the door beam is attached to the inner panel. Therefore, when the size of the door beam is significant, the material cost and weight of the door beam may undesirably increase.
When the door beam is made of a channel member or provided with a double hat-shaped cross section, the door beam contributes to the protection of the cabin interior at the time of a side crash by collapsing the cross section into a more flat shape and thereby absorbing the crash load. However, because the centroid given as a point where a geometrical moment of area becomes zero does not coincide with the direction of the force acting upon the door beam, the impact load of a frontal crash applies a bending moment to the door beam, and such a bending of the door beam prevents a favorable transmission of the crash load (load path) to a rear part of the vehicle body.
When the door beam is made of a pipe member, the centroid of the cross section thereof may be made to coincide with the direction of the crash load acting thereon as a result of a frontal crash by suitably supporting the two axial ends thereof so that the crash load of a frontal crash may not produce a bending moment. However, such a door beam does not so readily deform as a channel member when subjected to a lateral force, and is therefore unable to absorb the energy of a side impact of a side crash in a favorable manner.
Also, in either case, if the door beam is simply attached to the inner panel, the fore-and-aft axial load that acts upon the door at the time of a frontal crash is offset from the centroid of the cross section, and the efficiency of transmitting the fore-and-aft axial force is thereby impaired. The conventional door beams including those using a steel channel member tends to be incompletely collapsed, and the energy of a side crash cannot be absorbed so much as desired.